An epicyclic gear mechanism is one known type of gear mechanism. In the present specification, including the claims, an epicyclic gear unit is a gear unit having first and second gears which are rotatably mounted about respective shafts. A planet carrier is also rotatably mounted and includes at least one planet gear rotatably mounted on the planet carrier. Each planet gear engages both the first and second gears. There are thus three main elements to the epicyclic gear unit, namely the first and second gears and the planet carrier. The rotational speeds of these three elements are related. Whilst the planets can rotate relative to the carrier, no direct connection is made to the planet gears.
Epicyclic gear units can be formed in a variety of different ways. One type of epicyclic gear unit comprises a differential, with the first and second gears connected to two shafts which extend outwards in opposite directions and have a common axis.
Another form of epicyclic gear unit has the second gear formed as an annular gear. The axis of the first and second gears and the planet gears are then all parallel to one another, and the planet gears rotate in an annular space between the first gear and the second, annular gear.
Epicyclic gear units have numerous applications. However, in known arrangements, one of the three rotating elements is either fixed or capable of being fixed, so that a constant, fixed ratio is provided between the other two elements. One common application of epicyclic gear units is in the automatic transmissions of automobiles. Here, various brake bands are provided, to enable one of the elements to be braked or fixed, to cause transmission between the other two elements of the unit. By varying the elements which are fixed, one can obtain a variety of ratios between input and output shafts.
Automatic transmissions for automobiles usually include a torque converter, between the engine and the input shaft of the transmission itself. The torque converter is hydraulic, and includes an impeller and a turbine. The impeller is driven by the engine, and the turbine is driven by the fluid within the torque converter. The design of the various elements is such that, typically, the output shaft speed is less than the input shaft speed, and the torque at the output is a multiple of the torque at the input. The ratio between the input and output shafts varies, depending upon the operating conditions. However, whilst various modifications have been suggested to torque converters, they are always separate from the epicyclic gear units in the automatic transmission.
In my earlier patent application No. 082,618, there is disclosed an epicyclic gear apparatus, which is intended to provide a variable ratio between input and output shafts. However, this is only achieved by providing a third, control shaft which drives a third element of the epicyclic gear unit. As this can provide a variable ratio between the input and output, it does require the provision of a separate control shaft.